A typical direct drive servovalve includes a housing, a valve member (e.g., a spool, a poppet, etc.), a motor and a sensor. The housing defines a fluid pathway, and the valve member is configured to move within the fluid pathway between an open and closed position in order to control the amount of fluid flow within the pathway. The motor includes a stator which resides in a fixed position relative to the housing, and a rotor assembly which, if required, includes motor magnetics, a shaft and Hall sensor magnets. The rotor assembly (i) is configured to rotate relative to the stator and (ii) is linked to the valve member to control movement of the valve member within the housing. The sensor is configured to sense a present position of the servovalve (e.g., directly sense position of the valve member within the fluid pathway, directly sense a rotational orientation of the rotor assembly, etc.).
During operation, an electronic controller receives a command signal (e.g., from a user input device) directing the controller to operate the servovalve in a particular manner (e.g., increase flow, decrease flow, terminate flow, etc.). The controller also receives a position signal from the sensor thus enabling the controller to determine the present position of the valve member within the fluid pathway. The controller then sends a control signal to the motor based on both the command signal and the position signal (i.e., the controller outputs current through coils within the stator) to control the rotational orientation of the rotor assembly. As a result, the rotor assembly moves the valve member to a desired position within the fluid pathway thus controlling amount of fluid flow.
There are a variety of conventional approaches to sensing the present position of the servovalve. One conventional approach involves using a Hall effect transducer (i.e., a single analog Hall effect element) as the servovalve sensor. In this approach (hereinafter referred to as the conventional Hall effect transducer approach), the Hall effect transducer is initially positioned so that a null point is aligned with a maximum torque position of the motor. Then, as the rotor assembly moves to position the valve member within the fluid pathway, the Hall effect transducer outputs a signal which varies in voltage as the rotor assembly moves in either direction (clockwise or counterclockwise) from the null point.
Another conventional approach involves using a linear variable differential transformer (LVDT) as the servovalve sensor. In this approach (hereinafter called the conventional LVDT approach), a movable core of the LVDT moves in correspondence with movement of the valve member within the fluid pathway. One or more sensing coils of the LVDT then outputs a signal identifying the present position of the LVDT movable core and thus the present position of the valve member within the fluid pathway.